1. Field of the Invention
The present invention relates generally to interface circuits, and more specifically, to circuits that provide a programmable time delay substantially unaffected by process, voltage, and temperature variations.
2. Description of the Related Art
Interfacing with many source synchronous circuits (e.g., DRAM memories, and variations thereof) requires precise signal timing. For example, interfacing to DDR DRAM memories requires a clock signal and one or more data signals to be preferably adjusted so that the clock signal arrives at the DDR DRAM memory in the middle of a data valid window.
Traditionally, the time adjustment of clock signals has been accomplished by increasing the length of the clock line on the printed circuit board until the optimal timing is achieved. However, the use of extra wire reduces the quality of the clock signal and consumes already limited space on the printed circuit board. Another traditional method of delaying a signal has been accomplished with a phased lock loop (PLL) circuit. However, in addition to reliability and stability concerns, PLLs commonly require the use of a dedicated constantly running input clock signal, which may not be available, or which may not be cost effective to implement.
The use of on-chip delays, such as a PLL, suffers from another disadvantage in that they are susceptible to operating conditions. The precision by which a signal can be delayed, inside an integrated circuit for example, is limited by environmental and manufacturing conditions. Further, the speed of transistors utilized in integrated circuits depends on manufacturing (process), voltage, and temperature conditions. Because integrated circuits are expected to operate over a range of temperatures and voltages, the transistors themselves are unable to provide a constant delay when environmental conditions vary.
Accordingly, there is the need for a method and apparatus which provides a signal delay which remains substantially constant over process, voltage, and, temperature variations.